Atom probe field microscope having means for separating the ions according to mass

ABSTRACT

A field ion microscope capable of isolating and analyzing one or a few atoms of a specimen comprises a field ion microscope section including an emitter tip mounting the specimen to be examined, an apertured screen in the path of the beam of ions emitted by the specimen and means for adjusting the path traversed by the ion beam with respect to the aperture in the screen to cause a selected area of the beam to pass through the aperture and an ion detector positioned in the path of ions passing through the aperture. The path of the ion beam emitted by the specimen by the application of a high voltage may be altered by beam-deflecting means adjacent the beam path or by varying the angular disposition of the emitter tip with respect to the image screen, or the beam may be allowed to drift through a long tube and measured in its time-of-flight in each case using a detector of single particle sensitivity.

United States Patent [72] Inventor Erwin W. Mueller ilniyersity Park,Pa. [21] AppLNo. 822,362 5 [22] Filed May 7, 1969 [45] Patented Aug. 31,1971 [73] Assignee Research Corporation New York, N.Y.Continuation-impart of application Ser. No. 647,493, June 20, 1967, nowabandoned.

[54] ATOM PROBE FIELD ION MICROSCOPE HAVING MEANS FOR SEPARATING 'Il-IEIONS ACCORDING TO MASS 8 Claims, 5 Drawing Figs.

' [52] [1.8. CI 250/413, 250/495 [51] Int. Cl H0lj 39/34,

H01j 37/26 [50] Field of Search 250/4I.9, v

fi .-.9 fi. :9 8419- [56] References Cited UNITED STATES PATENTS2,206,415 7/1940 Marton 250/495 1) Primary Examiner-Archie R. BorcheltAssistant Examiner-A. L, Birch Attorney-Stowe & Stowell ABSTRACT: Afield ion microscope capable of isolating and analyzing one or a fewatoms of a specimen comprises a field ion microscope section includingan emitter tip mounting the specimen to be examined, an apertured screenin the path of the beam of ions emitted by the specimen and means foradjusting the path traversed by the ion beam with respect to theaperture in the screen to cause a selected area of the beam to passthrough the aperture and an ion detector positioned in the path of ionspassing through the aperture. The path of the ion beam emitted by thespecimen by the application of a high voltage may be altered bybeam-deflecting means adjacent the beam path or by varying the angulardisposition of the emitter tip with respect to the image screen, or thebeam may be allowed to drift through a long tube and measured in itstime offlight in each case using a detector of single particlesensitivi- PATENTED AUB3I I971 SHEET 2 OF 3 VACUUM PUMP ENTOR ERWNMUELLER ATTORNEYS PATENTEUAUG31 |97i SHEET 3 UF 3 INVEN'I OR ERWIN W.MUELLER ATTORNEY? ATOM PROBE FIELD ION MICROSCOPE HAVING MEANS FORSEPARATING TI'IE IONS ACCORDING TO MASS This application is acontinuation-in-part of my application Ser. No. 647,493 filed June 20,1967 now abandoned.

The invention relates to a field ion microscope capable of isolating andanalyzing one or a few atoms of a specimen under examination.

The field ion microscope (FIM), invented by applicant in 1951 (E. W.Miiller, Z. Physik I31, 136, 1951), is the most powerful microscopic deviceknown today, and the only one capable of showing the individual atoms asthe building blocks of the specimen surface. The microscope is nowwidely used as a research tool in physical metallurgy of the highermelting point metals. One of the serious limitations of the instrumentis its inability to discriminate the nature of the atoms seen on thespecimen. As an example, in an image with atomic details of aniron-tungsten alloy one cannot tell which of the atoms is an iron atom,and which is a tungsten atom. Such knowledge is highly desirable for thestudy of short range and long range order in alloys, or for determiningthe nature, composition and localization of certain atoms inprecipitates, or as solute or interstitial impurities.

On a much coarser scale identification of the constituents of amicroscopical specimen is being done in the electronmicroprobe analyzerin which the focused beam of an electron microscope causes the spotbeing investigated to emit X-rays, which are spectroscopically analyzedfor the wavelengths characteristic of each chemical element (R.Castaing, Electron Probe Microanalysis" in Advances Electronics andElectron Physics, Vol. 13, pp. 317-384, Academic Press, New York, 1960).The minimum spot size that can be investigated depends upon the spotsize of the electron beam, and to some extent upon the X-ray detectionsensitivity. In practice, the spot size is about one micron, and a pieceof solid matter of this size contains more than billion atoms. Somewhatgreater is the sensitivity of a scanning electron microscope, (A. V.Crewe, SCIENCE 154, 729, 1966), but still the resolution limits the spotsize to be analyzed to about 100 A., or a cluster of 30,000 atoms, andatom species discrimination is crude.

The purpose of the present invention is the provision of methods andapparatus for the analysis of a single atom or a few atoms in themicroscopic image that has been selected by the operator. The apparatusis appropriately called an atomprobe field ion microscope (atom-probeFIM).

One fonn of apparatus of the invention comprises a conventional fieldion microscope including an image screen which has a small aperture orprobe hole permitting a very small area of the image, usually the sizeof one atom, to be selected for analysis.

After an atom of interest has been pinpointed on the probe hole by asuitable image shifting device, the electric field strength at thespecimen is increased until the atom is torn off from its substrate byfield desorption or field evaporation. The atom leaves its site in theform of a singly or multiply charged ion and passes through the probehole. The atom is analyzed by determining its charge-to-mass ratio, e/m,by the use of a conventional mass spectrometric device that can be madesensitive enough for single particle detection. The invention includestime recording of the detector signal to provide a correlation betweenthe instant of field desorption of the atom to be analyzed and thesignal of the detector.

The principles of the invention will be further described with referenceto the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of an atom-probe field ionmicroscope including a magnetic mass analyzer;

FIG. 2 is a transverse section on line 2-2 of FIG. 1;

FIG. 3 is a diagrammatic representation of a modification in which anelectron mulitplier is positioned in the ion beam emerging from amagnetic mass analyzer;

FIG. 4 is a diagrammatic representation of a form of the invention inwhich'selection of an ion image from the ion beam of a field ionmicroscope is effected by varying the angular disposition of the ionbeam with respect to the image screen; and

FIG. 5 is a transverse section on line 5-5 of FIG. 4.

In FIGS. 1 and 2, the evacuated miscroscope section 10 contains theemitter tip specimen'12, which may be cooled by a cryogenic liquid 14,and kept at a high positive potential with respect to the screen 16.Between the tip and the's'creen are placed two pairs of deflectionplates 18 and 20, similar to the x-y beam deflection plates in aconventional cathode ray tube. Upon the application of a suitablevoltage plates 18 will electrostatically deflect the ion beam in thedirection parallel to the plane of the drawing. The other set of plates,20, operate in a direction normal to the plane of the drawing. Thus theion image beams can be deflected at will to place the desired atom spotof the ion beam image on the probe hole 22. Additional deflection platesmay be provided to assure a normal incidence of the beam on the hole.Observation or photography of the overall ion image is effected throughwindow 24. The microscope section of the device is filled with asuitable imaging gas, preferably helium, of a few microns or lesspressure.

The magnetic analyzer beyond the probe hole consists of a magneticdeflection field 26 normal to the plane of the drawing, and multichannelion detectors 28, 30 and 32. I The number of channels required dependson the desired discrimination. For the analysis of an alloy of knowncomposition, say iron-tungsten, three channels in-adjustable positionswill suffice. One channel 28 will be set where mass 4 of the imaginggas, helium, will appear. Channel 30 will be set on mass 56 where singlycharged Fe ions are to be expected, and channel 32 on mass 184 forsingly charged W ions. The amplified outputs of the three detectors willbe recorded on 34 as a function of time, while the electrical fieldapplied to the tip is slowly increased. The flux of I-Ie from theselected atom spot will suddenly cease when the atom in question isfield desorbed. The ion of the latter will appear as a single impact onthe proper detector. Noise discrimination is very good as only thedetector signal simultaneous with the cessation of the helium ion fluxis significant. A typical noise level even of an unsophisticatedelectronmultiplier detector is one per second, while the helium ion fluxis typically of the order of l0/sec., as indicated by the dense sequenceof spikes in the recorder diagram. The sweep time of the recorder is ofthe order of one second. Instead of using the step in the time recordingof the image gas ion beam it is also possible to record the beam of fastneutral atoms of the imaging gas which has its origin in charge exchangebetween the ion beam and the gas in the FIM vessel. This beam will notbe deflected in the magnetic analyzer, thereby simplifying the design.For increasing the correlation probability, both the image gas ion andneutral beam may be time-recorded to mark the event of field desorptionof the specimen atom in question.

In a modified embodiment of the invention a multichannel detector can beused in the image plane of the mass separation magnet. The detectorsection of an atom-probe FIM with this feature is shown in FIG. 3. Herethe image plane of the analyzer magnet 26 is occupied by a Venetianblind-type electron multiplier 36 which sends its signals to an outputscreen 38 by means of lens 40. This screen can be viewed or can bephotographically recorded on a drum-type film camera 42 to record anunknown ion impact at the instant of cessation of the image gas ion flux44.

A different mode of ion discrimination is shown in FIGS. 4 and 5. Thevacuum vessel 50 of the FIM section includes a bellows device 52 whichmay be used to tilt the specimen tip 54; in any desired direction.Preferentially the tilting motion of the tip is constrained by aspherical joint 54 so that the imaged tip and associated auxiliary ringelectrode 56 retains its position in space. With this feature any partof the ion image as it appears on the phosphor screen 58 and is viewedthrough the window 60 can be brought onto the probe hole 62. Normalincidence of the probe hole beam is thus assured at all times. The massdiscrimination of the ions after passing the hole is effected by theconventional method of time-of-flight mass spectrometry. The analyzersection beyond the probe hole consists essentially of a drift tube 64typically a few to about 50 inches in length and difi'erentially pumpedthrough the vacuum system 66. An ion detector 68 at the end, havingsingle particle sensitivity, will pick up the arriving ions and theoutput of the detector is recorded on a fast oscilloscope as a functionof time. The sweep time of the oscilloscope is typically of the order ofl to 100 microseconds. The scope sweep is triggered by a nanosecondsrisetime pulse given onto the auxiliary electrode 56 or is superimposedon the DC tip voltage. This pulse also causes the atom under study to betorn off from the tip specimen. A few microseconds later, the particlewill arrive at the detector and will be recorded. The time-of-flight isa measure of the e/m ration which gives the desired atom identification.The arrival at the detector of the imaging gas ions is not disturbing,as such events, occurring in an average time spacing of 100 microsecondsor more, will only rarely be recorded during the short sweep period.

It may be advantageous to interrupt the supply of imaging gas, usuallyhelium, just before the desorption process is initiated. This can bedone by closing a valve in the supply line 69. During adjustment of theimage, the required helium gas pressure is maintained by either valvingoff the pumping lines,

or by operating in a dynamic gas supply mode. (B. Waclawski and E. W.l\/Iiiller, J. Appl. phys. 32, 1472, 1961 In the latter case, themicroscope has the gap between he upper part 50 and the lower part ofthe microscope section much narrower than the cross section of the upperpumping system 67. This can be effected by providing the ring electrode56 with a small orifice, near the center of which is the specimen tip54. As a result, the helium gas pressure in section 50 and at thevicinity of the tip will be higher than in the rest of the instrument.

The probe hole in the screen of the field ion microscope section willusually be of a size equivalent to the to the apparent diameter of theatom selected for analysis. This size varies with the tip radius and themagnification of the microscope. The microscope therefore preferablyincludes means for adjusting the size of the probe hole duringobservation. This may be done by selecting a desirable size from anarray or a revolver plate of various size holes, or using an iristypevariable aperture. For certain applications, such as the study ofordered alloys, it may be desirable to make the probe hole large enoughto cover an area of several adjacent atoms that will be field desorbedsimultaneously. The probe hole may also have an oblong shape of variablesize and means to change the direction of the probe slot with respect tothe ion image symmetry. This allows one to place the probe slot along achain of atoms which are to be analyzed simultaneously.

It is not essential that the area of the selected ion beam be determinedby the probe hole 62 of the apparatus of FIG. 4 as the hole 62 may belarger and a field of view limiting aperture placed further along thebeam path between phospher screen 58 and the detector 68. If such afield of view limiter is used, the image screen may be unapertured andbe tilted out of the path of the ion beam when the selected portion ofthe beam is to be analyzed.

It will be understood that any combination of the features of thesystems described above may be applied in the utilization of the.invention. For example, the magnetic analyzer type atom-probe FIM may beequipped with the bellows type of image point selection as well as withthe differential pumping scheme in the analyzer section, and vice versa.

I claim:

1. An atom-probe field ion microscope comprising a microscope sectionincluding an emitter tip mounting a specimen to be examined, means forintroducing an ionizable gas into the section, an image screen in thepath of the beam of ions emitted by the specimen apertured to pass theimage of a single atom of the emitter tip, means for adjusting the pathtraversed by the beam with respect to he aperture in the screen to causea selected area of the beam to pass through the aperture and means forincreasing the electrical field strength at the emitter tip to efi'ectemission of ionized atoms of the emitter tip material and an iondetector positioned in the path of ions passing through the aperture.

2. An atom-probe field ion microscope as defined in claim 1 includingpaired deflector electrodes adjacent the path of the ion beam emitted bythe specimen for varying the direction of travel of the ion beam.

3. An atom-probe field ion microscope as defined in claim 1 wherein themounting means for the emitter tip includes a flexible bellows forvarying the angular position of the emitter tip with respect to theapertured screen.

4. An atom-probe field ion microscope as defined in claim 1 including anion detector channel detecting ions of gas atoms ionized at the emittertip and at least one further channel detecting ions of desorbedQpecimenatoms passing through the aperture.

5. An atom-probe field ion microscope as defined in claim 1 includingmeans establishing a magnetic field transversely of the path of ionspassing through the aperture and at least one ion detector channel inthe path of ions deflected by the magnetic field.

6. An atom-probe field ion microscope as defined in claim 1 includingmeans for pulsing the field on the emitter tip to effect the desorptionof atoms from the specimen.

7. An atom-probe field ion microscope as defined in claim 6 includingmeans for relating the time of a pulse and the time of detection of anatom desorbed by such pulse to determine the e/m ratio of such atom.

8. An atom-probe field ion microscope as defined in claim 1 includingmeans for altering the size, shape or location of the effective beamlimiting aperture.

1. An atom-probe field ion microscope comprising a microscope sectionincluding an emitter tip mounting a specimen to be examined, means forintroducing an ionizable gas into the section, an image screen in thepath of the beam of ions emitted by the specimen apertured to pass theimage of a single atom of the emitter tip, means for adjusting the pathtraversed by the beam with respect to he aperture in the screen to causea selected area of the beam to pass through the aperture and means forincreasing the electrical field strength at the emitter tip to effectemission of ionized atoms of the emitter tip material and an iondetector positioned in the path of ions passing through the aperture. 2.An atom-probe field ion microscope as defined in claim 1 includingpaired deflector electrodes adjacent the path of the ion beam emitted bythe specimen for varying the direction of travel of the ion beam.
 3. Anatom-probe field ion microscope as defined in claim 1 wherein themounting means for the emitter tip includes a flexible bellows forvarying the angular position of the emitter tip with respect to theapertured screen.
 4. An atom-probe field ion microscope as defined inclaim 1 including an ion detector channel detecting ions of gas atomsionized at the emitter tip and at least one further channel detectingions of desorbed specimen atoms passing through the aperture.
 5. Anatom-probe field ion microscope as defined in claim 1 including meansestablishing a magnetic field transversely of the path of ions passingthrough the aperture and at least one ion detector channel in the pathof ions deflected by the magnetic field.
 6. An atom-probe field ionmicroscope as defined in claim 1 including means for pulsing the fieldon the emitter tip to effect the desorption of atoms from the specimen.7. An atom-probe field ion microscope as defined in claim 6 includingmeans for relating the time of a pulse and the time of detection of anatom desorbed by such pulse to determine the e/m ratio of such atom. 8.An atom-probe field ion microscope as defined in claim 1 including meansfor altering the size, shape or location of the effective beam limitingaperture.